The invention relates to devices and methods for percutaneous endocardial revascularization of the heart using a flash lamp-pumped laser.
Transmyocardial revascularization (TMR) is a surgical treatment for cardiovascular disease. Present TMR procedure is an open chest technique (thoracotomy) that uses a laser beam to drill holes through the myocardium, typically into the left ventricle. These holes or channels extend through the entire heart wall thickness from the outside through to the ventricle. The openings of the channels on the outside surface of the heart heal due to external pressure from the surgeon, but the channels are believed to remain open on the inside, allowing blood to enter the heart wall tissue from the ventricle.
In another approach ELR can be performed using a catheter introduced percutaneously so that the tip of the catheter is inside a chamber of the heart, typically the left ventricle, where the holes or channels can be created from the inside toward but not through the outside of the heart. The channels are drilled with a laser beam introduced through the catheter.
Certain problems are presented when laser revascularization is done on a beating heart. A beating heart presents a moving target, which can make it difficult to accurately and consistently form channels of a desired depth and size. The heart also is extremely sensitive to a laser pulse at certain times during its cycle. A laser pulse striking the heart at the T time of an electrocardiogram (ECG) signal could cause the heart to fibrillate and result in heart failure. While one could stop the heart during the process of TMR, this poses other risks to the patient and complicates the operating procedure. The heart must be cooled and the patient connected to a heart-lung machine.
However, the risk of inducing a beating heart to fibrillate is greatly reduced when the laser is fired only during the refractory period of the heart cycle between the R and T waves of the ECG signal. An additional benefit of firing the laser only between the R and T waves is that this is the period of the heartbeat cycle during which the heart is most still and channels can be formed most accurately. Co-owned U.S. Pat. No. 5,125,926, to Rudko et al., describes a heart-synchronized pulsed laser system that fires a laser only during the refractory period of the heartbeat cycle. The '926 patent discloses an open chest procedure using an articulated optical arm or a fiber optic element to deliver the laser beam to the outer surface of the heart. The laser described in the '926 patent is a pulsed 50 Joule CO.sub.2 laser, which produces a laser beam with a wavelength of about 10.6 .mu.m. In a procedure using the articulated arm, a channel penetrating through the myocardium can be formed during a single firing of the laser, in a period of about 50 ms or less, during the refractory period of the heartbeat cycle.
U.S. Pat. No. 5,389,096, to Aita et al., discloses a percutaneous TMLR procedure in which a steerable heart catheter is guided from the femoral artery via the abdominal artery into the left ventricle. The laser energy is delivered through the working channel of the catheter by a fiber optic delivery system. The '096 patent describes a system employing a holmium laser, which produces a beam having a wavelength of about 2.1 .mu.m. This wavelength is desirable for TMLR because it is strongly absorbed by heart tissue, and so is efficient for forming channels. It is also a wavelength that can be passed through a fiber optic element.
Presently available flash lamp-pumped lasers typically include a capacitor bank that discharges through a thyristor to energize a flash lamp. The capacitor bank, which typically stores a couple of hundred Joules of energy and has a capacitance of a couple hundred .mu.f, completely discharges when the thyristor is enabled. The flash lamp only flashes during the discharge. This typically takes no more than a few hundred microseconds (.mu.s). Because the laser is pumped only during the operation of the flash lamp, the laser beam produced by the laser rod in the prior art systems is also limited to a pulse duration of only a few hundred .mu.s. Moreover, prior art flash-pumped holmium lasers used for ELR could only produce pulses with a maximum energy of about 3-5 Joules (J) at a power of about 10 KW.
The energy necessary to form a 10 mm channel in the heart, which is a desirable depth for revascularization, is typically greater than 5 J, for example, 10-20 J. However, due to limitions of the optical coatings of components in the laser cavity, these short pulses, having a duration of a few hundred .mu.s, cannot be generated with much higher energies without damaging the laser. Therefore, as described in the '096 patent, several lower energy pulses applied at a frequency of at least 2 Hz are required to form each channel, the pulses being applied over the course of more than one heartbeat. The '096 patent also describes employing an excimer laser. However, an even greater number of laser pulses are needed with the excimer laser, again due to limitations of the fiber optic system.
A variety of problems arise when more than one pulse is employed to form a channel through the heart wall. After each laser pulse, the channel being formed can fill with blood, which absorbs much of the energy of subsequent pulses, making the subsequent pulses less efficient in deepening the channel. A beating heart can also move between pulses, and the surgeon must be particularly careful to direct the laser beam of each pulse along the same line. If the laser pulses are applied during different heartbeats, these problems can be multiplied. Moreover, due to the cavitation effects during ablation, a high risk of catheter displacement from the endocardium is presented.